When I downlaoded the headers for PU, it appeared I was connected to giganews. Is PU actually GN in disguise ( in other words GN doing business also as PU to provide an alternative to itself) or are they just sharing headers, etc?

I believe they're a reseller, yes. They bulk buy from Giga and offer different options for subscription. I'm pretty sure Giga resells to a number of providers, some of them ISPs. ISPs themselves work this way - a smaller company will bulk buy from the bigger fish and resell aimed towards a different market. There's nothing in disguise - it's quite legit and all above-board.